Simon Westerlund westerlund

Analysis of Apple’s rubber band scrolling

January 6, 2013

I recently saw a post on Twitter from @chpwn that described the alogorithm that Apple uses for its “rubber band” or “bungee” scrolling.

b = (1.0 – (1.0 / ((x * c / d) + 1.0))) * d

	extension View {
	// https://www.avanderlee.com/swiftui/conditional-view-modifier/
	/// Applies the given transform if the given condition evaluates to `true`.
	/// - Parameters:
	/// - condition: The condition to evaluate.
	/// - transform: The transform to apply to the source `View`.
	/// - Returns: Either the original `View` or the modified `View` if the condition is `true`.
	@ViewBuilder func `if`<Content: View>(_ condition: Bool, transform: (Self) -> Content) -> some View {
	if condition {
	transform(self)

	//
	// SwiftUICollectionView.swift
	// Sentry
	//
	// Created by Simon Westerlund on 2019-12-29.
	// Copyright © 2019 Simon Westerlund. All rights reserved.
	//

	import SwiftUI

	struct FontSizesHelper {
	static func dynamicSize(for originalSize: CGFloat, category: UIContentSizeCategory = UIApplication.shared.preferredContentSizeCategory) -> CGFloat {
	let modifyBy: CGFloat = {
	switch category {
	case .extraSmall: return -3
	case .small: return -2
	case .medium: return -1
	case .large: return 0
	case .extraLarge: return 2
	case .extraExtraLarge: return 4

	func orThrow<E: Error>(_ errorClosure: @autoclosure () -> E) throws -> Wrapped {
	guard let value = self else {
	throw errorClosure()
	}

	return value
	}

	// usage
	orThrow(UnboxError.invalidData)

	// based on this answer 'http://stackoverflow.com/a/36016798'
	class LeftAlignedCollectionViewFlowLayout: UICollectionViewFlowLayout {

	override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
	let attributes = super.layoutAttributesForElements(in: rect)

	var leftMargin = sectionInset.left
	var maxY: CGFloat = -1.0
	attributes?.forEach { layoutAttribute in
	if layoutAttribute.frame.origin.y >= maxY {

	// A boiler plate operation to be used as a subclass
	//
	// Created by Simon Westerlund on 2016-11-07.
	// Copyright © 2016 Simon Westerlund. All rights reserved.
	//
	import Foundation

	final class BaseOperation: Operation {
	private var _isFinished = false {
	willSet { willChangeValue(forKey: "isFinished") }

	extension String {
	func pirated() -> String {
	let consonants = "bcdfghjklmnpqrstvwxyz"

	return characters.map({ char -> String in
	let character = String(char)
	if consonants.contains(character.lowercased()) {
	return "\(char)o\(character.lowercased())"
	}
	return character